1. Field of the Invention
The present invention generally relates to a method for reducing a dielectric constant of a film, particularly to such a method for a film containing a pore generator (“porogen”) using direct plasma or capacitively-coupled plasma of hydrogen.
2. Description of the Related Art
These days, technology in the field of semiconductor manufacturing equipment shows remarkable improvement, achieving high integration, high speed, and high capacity, which makes wiring more fine. The more fine wiring becomes, the higher capacity the wiring requires, especially if the wiring is configured in multiple layers. As a result, such fine wiring causes signal delay, and increases electricity consumption. To solve these problems, and to reduce the amount of required capacity, insulating films having low dielectric constant have been used for multilayer wiring.
Taking into consideration how future semiconductor manufacturing equipment will be produced, to meet demand, the dielectric constant (∈) should be at least lower than ∈=2.4. To achieve this, U.S. Pat. No. 7,049,247 and U.S. Pat. No. 7,208,389, for example, disclose a method for forming low dielectric constant films with dielectric constant (∈) lower than ∈=2.7. The first step in the method shown above is to mix organic silane or organic siloxane, with organic material called a “porogen”. Organic silane and organic siloxane are the materials forming a basic matrix of a thin film. The porogen forms pores afterwards. The second step is to form a thin film by causing chemical reactions in plasma. Next, in the third step, the thin-film is exposed to heat or UV radiation (UV-curing) to remove the porogen so as to leave pores in the thin-film and to reduce the density of the film. As a result, the value of ∈ should be lower than ∈=2.7. It is expected that producing films with even lower dielectric constant can easily be achieved by adding more porogen to the film. By doing so, more porogen will be removed in the UV-curing process, and as a result, more pores will be left in the thin-film.
The method above which uses porogen in the film forming process has the benefit that it can control the value of the dielectric constant by changing the ratio between the porogen and organic silane/organic siloxane, or the conditions under the UV-curing process. On the other hand, the present inventor recognizes problems mentioned below. Since pores are created in the process of exposing a film to heat or UV radiation and removing the porogen, lower dielectric constant is expected to be achieved by adding a larger amount of porogen to the film. However, even though the size of one molecule of porogen is small, if the amount of the porogen is large, the molecules of porogen are likely to be close to each other and cause aggregation. As a result, the porogen will be removed as a lump of molecules, which forms a large pore in the film after removal. With this pore, not only can the mechanical strength of the film no longer be maintained, but also it causes obstacles in the process of producing semiconductor manufacturing equipment, because some substances such as gas, metal molecules, and liquid chemicals can easily penetrate into the pore. Further, it is expected that it is possible to achieve higher mechanical strength of the film by extending UV-curing time to the most appropriate time. However, by doing so, it also causes a greater degree of film shrinkage, which results in producing films with higher dielectric constant.
Any discussion of problems and solutions involved in the related art has been included in this disclosure solely for the purposes of providing a context for the present invention, and should not be taken as an admission that any or all of the discussion were known at the time the invention was made.